Clay differs from earth or soil in that it possesses certain
characteristics which these do not possess. Its distribution is very
wide but for the most part it lies concealed from view. In certain parts
of the country it is so abundant that it breaks through the surface or
is exposed as an outcrop but usually it is covered by the soil which
supports vegetation. Unless the subsoil consists of sand it is easy to
expose a clay by plowing or digging with a spade. It usually appears as
a greenish or bluish substance of close and uniform structure. The
texture is sometimes smooth but more often numerous small stones are
found imbedded in the mass. Such clays as are commonly found can be used
for the manufacture of some kind of pottery but in the great majority of
cases the ware will be red when fired because the clay contains a
proportion of oxide of iron. A pure clay does not contain this and
therefore becomes white or nearly white in the kiln.
Pure clay, known as clay base or clay substance forms a part of all
natural clays though sometimes only a small part. It consists of silica,
alumina and water in a state of combination and is thus known as a
hydrous aluminium silicate. While this substance is very common as an
ingredient of ordinary clay, it is rarely found alone or uncontaminated.
Commercial or workable clays may be said to consist of clay base and
sand, with or without other impurities such as lime and oxide of iron.
For working purposes it may be granted that the potter has to deal with
a mixture of clay and sand. But sand is not a definite expression. It
may vary both physically and chemically within wide limits. The physical
nature has to do with condition, the chemical with composition. Thus a
sand may be almost as coarse as gravel or as fine as the clay itself. It
may be a pure quartz sand or it may be a crushed rock of almost any
composition. The former is known as quartz, the latter as feldspar or
feldspathic sand because it approaches in composition the group of
minerals known as feldspars. Each of these ingredients, clay, quartz and
feldspar, has an important part to play in the transformation of clay
into pottery. Few of the clays used in making white pottery possess
these ingredients in the correct proportions so that it becomes
necessary to make a mixture in which the necessary proportions will be
found.
For successful pottery making three properties are demanded in a clay.
First, plasticity. Without this, clay could not be shaped at all. It
constitutes the obedience of a clay to the forming influence whether
hand or mold. The necessity for this quality may be illustrated by the
proverb "Making ropes of sand" as an example of the impossible. Sand,
possessing no plasticity, cannot be shaped or made to hold together.
The second property is porosity. A clay which exhibits a high degree of
plasticity can be easily shaped but it cannot be safely dried. The water
of plasticity cannot escape and therefore the clay warps and cracks. The
function of porosity is to prevent this. A porous clay permits the water
to escape freely and the clay can be dried without damage. This
condition is produced by the admixture of sand or by the presence of
sand in a natural clay. A coarse sand is more effective than a fine sand
but a sand that is too coarse will interfere with delicate working while
a sand that is too fine approximates the action of the clay itself and
produces a substance which is dense rather than porous. Porosity is
therefore the reverse of plasticity and these two properties must be
adjusted so as to balance each other.
The third necessary property is commonly known as vitrification but
could be better named "densification" because complete vitrification is
not attained in ordinary clay wares. This property may be defined as
that which causes a clay to yield to the action of a high temperature so
that the result is a ware, more or less dense, which is hard, durable
and sonorous. With this there must be coupled a certain amount of
resistance to heat treatment so that the pottery does not fuse or
collapse during the firing. Here also is found the need for adjustment.
The clay must yield to the fire but not completely. It must resist but
not entirely.
Plasticity is due to the clay base. Not only to its quantity but to its
quality also. Some forms of clay in which clay base predominates are not
plastic because the clay base itself is coarse grained. Other forms with
less clay base present are plastic because this ingredient is fine
grained and tough. Pure clay base is also highly resistant to fire and
therefore contributes to the refractoriness of the mass.
Porosity is caused by the sand in the clay. Any kind of sand will
produce porosity but the effect differs with the condition of the sand.
Coarse sand is more effective than fine sand. More sand will, of course,
cause greater porosity.
Vitrification or densification is due to the feldspar or fusible sand.
This also varies with the condition. A fine-grained feldspar will
produce vitrification more easily than the same amount of coarse
feldspar.
Certain substances are available for use in pottery mixtures, which
possess one or other of the necessary properties in high degree so that
they will impart these properties to a mass to which they are added.
Kaolin or china clay is usually fine, white, and refractory. Some
kaolins are rather plastic but most of them are "short" in working and
rather tender. For the production of a white ware kaolin is
indispensable. No other ingredient will afford the pure white color
which is sought after in porcelain and china.
Ball clay is very plastic, easily vitrified, but is not white. The color
varies from a cream to a gray. The use of a ball clay is therefore
limited in white wares because it will spoil the color. For wares in
which a light cream color is not objectionable ball clays are valuable
and almost indispensable.
Stoneware clay is usually a rather plastic clay which contains a good
deal of sand, hence stoneware clays can be used for certain classes of
ware without admixture. A rather high temperature is required for most
of these clays, though occasionally one can be found which will become
dense at the fire of a studio kiln. The clays sold by the Enfield
Pottery Company and by the Western Stoneware Company are of this type.
Ground flint is a necessary ingredient in almost all pottery. It aids in
the porosity of the clay and enables the mixture to be adjusted to fit a
special glaze.
Ground feldspar is also necessary. Like flint it aids in the porosity of
the unburned clay but unlike flint it produces density in the firing.
By a proper adjustment of these ingredients a clay can be composed which
will meet the special requirements of the worker.
In order to ascertain the properties of any given clay certain simple
tests may be made and every clay-worker should know how to do this
because one cannot be too well informed as to the materials to be used.
First, water of plasticity. A certain portion of the clay, dried and
powdered, is weighed out. It is convenient to weigh in grams and to
measure in cubic centimeters because in this way calculation is easy.
The scales and weights are described in the chapter on glazes. For
measuring the water a glass vessel called a graduate is used. One
holding a hundred cubic centimeters and graduated in centimeters and
tenths can be obtained from a dealer in chemical supplies. One hundred
grams of clay is weighed out and transferred to a glass slab. The
graduate is filled with water to the one hundred mark. Some of this
water is then poured on to the clay, adding little by little as needed
until the whole can be worked into a stiff mass of the proper
plasticity. The quantity of water used is then carefully noted by
observing how much is left in the graduate. Suppose, for instance, 70
cubic centimeters are found remaining, the hundred grams of clay has
absorbed thirty c.c. of water and as one c.c. of water weighs one gram
the clay has taken just 30 per cent. This amount is important because it
is one of the best indications of plasticity. A very plastic clay may
need 40 per cent, a non-plastic clay may be satisfied with 25 per cent.
Second, shrinkage. The mass of plastic clay is now transferred to a
plaster bat and rolled or pressed out into a smooth slab about 12
centimeters long. Here again the centimeter is used in preference to the
inch as being more easily calculated. A faint line is ruled on the clay
slab and two fine scratches are marked exactly ten centimeters apart.
The edges are trimmed and the excess clay made up into three or four
small pieces which are to be fired in different parts of the kiln as
tests for density. When the clay slab is dry the distance between the
marks is measured and noted. The ten centimeters being divided into one
hundred millimeters, each millimeter of shrinkage means one per cent.
After firing, a second measurement is made and the differences are noted
as dry shrinkage and fire shrinkage respectively.
Third, firing. The slab with the measurement upon it is set in the kiln
in the place where the clay wares are to receive the first or biscuit
fire and the small pieces are arranged in different places so as to
secure as many different conditions as possible. The position of each
should be carefully recorded. After firing, the marks on the slab are
measured as already described and note is taken of any warping of the
piece. The color is also recorded. The small pieces should be tested for
porosity or absorption of water but this is rather a delicate operation
and needs a particularly sensitive balance. Generally it will suffice to
use a wet sponge or to dip each piece into water, removing it quickly
and noting carefully the rate of speed at which the water is absorbed.
If the water should be scarcely absorbed at all a line of ink may be
drawn upon the pottery with a pen, the piece being perfectly dry. In a
fully vitrified ware the ink can be washed off, leaving scarcely a mark
but the test is quite sensitive and with a little practice will afford
an excellent means of comparing the density of different clays or of the
same clay at different temperatures.
Fourth, glazing. It is well to have ready a small supply of a standard
clear glaze. Each of the test pieces should be covered with this in a
rather thin coat and then they should all be fired again, this time
close together so that they will receive the same heat treatment. This
will enable one to determine what degree of fire for the clay will best
suit the glaze.